A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to an evacuated process enclosure housing the substrate and react to form a thin film over the surface of the substrate. An example of a CVD process used to deposit an aluminium layer on a substrate is MOCVD (metal organic chemical vapour deposition), in which an organoaluminium precursor is supplied to the process enclosure from a bubbler, the precursor being entrained within a carrier gas, such as nitrogen or argon, conveyed to the bubbler. A hydrogen reducing gas is also supplied to the process enclosure for reducing the precursor. The process enclosure is evacuated, and heated to a deposition temperature, generally less than 500° C., at which the precursor decomposes and aluminium is deposited on to the substrate.
Another known technique for depositing a thin film on a substrate is a physical vapour deposition (PVD) process, in which the material to be deposited is converted into a vapour. This vapour is transported across an evacuated process enclosure from its source to the substrate, where the vapour condenses to form a thin film on the substrate.
In such deposition processes, the residence time of the deposition gases in the processing enclosure is relatively short, and only a small proportion of the gas supplied to the enclosure is consumed during the deposition process. Consequently, much of the deposition gas supplied to the process enclosure is exhausted from the enclosure together with by-products from the deposition process, and conveyed to a secondary vacuum pump used to evacuate the process enclosure. A turbomolecular pump, capable of evacuating the process enclosure to a low pressure, is often used as the secondary pump.
A process tool usually comprises a plurality of process enclosures, in which similar or different processes may be conducted at any given time. For example, in addition to one or more deposition enclosures, the process tool may comprise one or more etch enclosures within which features are etched within the substrate and/or within the thin film deposited on the substrate. Separate secondary pumps are generally used to evacuate the enclosures. In order to reduce the complexity, footprint and power requirements of the overall pumping arrangement while improving reliability and costs associated with the pumping arrangement, it is preferable to provide a single primary pump to back a number of separate secondary pumps.
Light gases, that is, gases such as hydrogen having a low relative molecular mass, are seen as contaminants for a number of deposition processes. For example trace quantities of these gases in the process enclosure in which a physical vapour deposition (PVD) is being performed can have a negative impact on the properties of the layer being deposited on the substrate.
There are a number of potential sources of such contamination, including cracking of moisture, and contamination of the gases supplied to the process enclosure. As another example, hydrogen that is being pumped from a first process enclosure by one secondary vacuum pump can migrate backwards through the secondary vacuum pump used to evacuate a second process enclosure when a single primary pump is used to back both secondary vacuum pumps, and particularly when the secondary pumps employ turbomolecular pumping mechanisms.